The present invention relates to a mobile station of a radio communication system which can enter into direct communication with at least one other mobile station.
A mobile station of a radio communication system is a mobile station which operates according to the specifications associated with this radio communication system. In the rest of the description, the radio communication system in question is the mobile telephony system known as GSM (Global System for Mobile communications). It will be understood that the present invention is not limited to this radio communication system but rather encompasses all systems for which the specifications are substantially equivalent.
The essential specifications of the mobile telephony system known as GSM will be recalled below. This system is a time division multiple access (TDMA) system. In this system, the information exchanged is generally transmitted in the form of digital data within time intervals (time slots: the term “slot” will be used to denote such a time interval since this is the term used by the person skilled in the art) The frame structure of the GSM system will be recalled below with reference to FIG. 1.
A TDMA frame is a set of eight successive slots s0 to s7. The slots of a given equal order of several successive TDMA frames together form what is known as a multiframe of a TDMA frame. For example, in FIG. 1, the slots s0 of the successive frames form the multiframe M0 and the slots s2 of the successive frames form the multiframe M2. There are two multiframe structures: one which groups together 26 slots and one which groups together 51 slots.
Superframe and hyperframe structures are also defined in the specifications of the GSM system, but these are not considered here.
For the transmission of user data, a mobile station has two slots which are respectively transmitted on two different carriers: one assigned to uplink transmission (mobile station to base station) and the other assigned to downlink transmission (base station to mobile station). These data may be useful data and they are then transmitted in logical channels known as TCH (Traffic CHannel), or they may be signalling data and they are then transmitted in logical channels known as SDCCH. Logical control channels may accompany these logical channels: SACCH (Slow Associated Control CHannel) and FACCH (Fast Associated Control CHannel).
A beacon channel is also implemented in order to allow a mobile station to connect to the most favourable base station and synchronise with the latter. This beacon channel is transmitted, by each base station of the system, on a particular frequency which is selected from the set of carriers attributed to the mobile station in question. It comprises the following logical channels transmitted in broadcast mode: the channel FCCH (Frequency Correction CHannel), the channel SCH (Synchronisation CHannel) and the channel BCCH (Broadcast Control CHannel). The channel FCCH is transmitted in the slot s0 of some predefined frames, for example the single frames 0, 10, 20, 30 and 40 of a multiframe containing 51 frames (see the slots s0 of the multiframe M0 marked with an X in FIG. 1). The channel SCH is transmitted in the slot s0 of the frame following that in which the channel FCCH has been transmitted, for example in the single frames 1, 11, 21, 31 and 41 of a multiframe containing 51 frames (see the slots s0 of the multiframe M0 marked with a + in FIG. 1).
A set of common logical channels is also provided in order to make it possible in particular to set up calls and allocate dedicated logical channels (TCH, SDCCH). These are not considered here.
When it is powered up, a mobile station examines the set of carriers that it receives in order to search therein for the signal which corresponds to a channel FCCH. Once this signal has been found, it can on the one hand adjust the frequency of the carrier received, but also on the other hand deduce therefrom a temporal position, so that it can align itself with the frame received on this carrier. In the next frame, it can then read, in the corresponding slot s0, the channel SCH in which it will find all the information required to perfect its alignment and its synchronisation with the frames. It can then read the channel BCCH which will provide it with information about the characteristics of the base station with which it is synchronised but also those of the neighbouring base stations.
In this state, the mobile station can start the setting up of a connection, normally by accessing the common channel RACH, or respond to a paging signal present on the channel PCH.
Once a connection has been set up for the transmission of data, the mobile station receives the common channels SCH, FCCH and BCCH not only from the base station to which it is connected but also from the neighbouring base stations. It can thus, according to the circumstances, connect to the base station which is most favourable at the time in question.
It will be noted that the logical channels of the beacon channel FCCH, SCH and BCCH are downlink channels, that is to say they are exclusively transmitted in broadcast mode by a base station to mobile stations.
Furthermore, it must be pointed out here that the frequencies assigned to the uplink direction are not in the same frequency band as those assigned to the downlink direction. More specifically, the frequency assigned to the uplink direction is shifted by a predetermined value with respect to the frequency assigned to the downlink direction.